Innumerable techniques exist for producing cavities in materials, most involving some mechanism for melting, abrading, etching, or machining the material, or molding it so as to form a cavity in a desired shape and position. However, producing an extremely long, narrow cavity or passage within a component, e.g., a cavity having a cross-sectional dimension that is less than a millimeter and a length measured in meters, represents a much more difficult task for which a solution has not previously been available. Clearly, conventional techniques for forming an elongate cavity using a drill or an energy beam to vaporize the material are useless in producing a cavity with such dimensions.
A partial solution to the problem seems to be disclosed in U.S. Pat. No. 3,630,799. This patent teaches a method for forming a plurality of spaced-apart passages within a supporting medium. According to this method, a laminate structure is formed with a plurality of wires embedded within it, positioned where the elongate passages are desired. The wires are removed from the laminate by subjecting them to a pulling force, which causes elongation of the wires and decreases their overall diameter, purportedly freeing them so that they are easily removed from the laminate. Not disclosed in the patent is the type of material from which the wires are made, or any limitations on the diameter or length of the elongate passages that can be formed using this technique. However, it is likely that the technique is limited to forming passages much less than a meter in length. Conventional metal wire is subject to work hardening as it is plastically deformed by stretching it lengthwise. As the wire stretches, it thus becomes brittle and breaks when the frictional drag developed between the material in which it is embedded and the wire increases beyond the tensile strength of the wire.
In U.S. Pat. No. 4,361,533, a related method is used to mold a fiberglass tennis racket frame having two elongate cavities formed within it. The frame is constructed from two small diameter fiberglass tubes formed by rolling fiberglass strips around elongate silicone rubber slugs, which are approximately six millimeters in diameter and about 64 centimeters in length. The small diameter fiberglass tubes are inserted into a larger diameter fiberglass cylinder, and the larger cylinder is pinched around and between the tubes to form a tube having a figure-8 cross-sectional shape. The component is then placed into a curing mold and heated, causing the silicone rubber to expand, so that it pushes the surrounding fiberglass into contact with the mold. Once the molded frame has cooled, the silicone rubber slugs are withdrawn by pulling on them lengthwise, thereby reducing their diameter so that they self-free from the cured frame.
Both of the prior art techniques discussed above offer only a partial solution to the problem initially posed. As is often the case in technology involving the fabrication of components, both methods are capable of forming elongate cavities of only limited length and relatively large cross section. The limitations are matters of degree. Clearly, neither conventional wire nor silicone rubber slugs can be successfully employed as taught by the references, to form passages that are less than a millimeter in diameter and several meters in length, because conventional wire material and silicone rubber of such small diameter and length would break before being pulled free of a component.
A different technique for forming elongate cavities in a component is taught in U.S. Pat. No. 4,675,061. This technique relies upon an unusual characteristic of a class of metal alloys referred to as shape memory metals. Such metal alloys revert to a memory shape when heated above a critical temperature, causing them to change from a martensitic state to an austenitic state. The patent shows how hollow tubular, trapezoidally-shaped cores of memory metal alloy can be molded between a planar base layer and an overlying layer of fiberglass or other type of composite. The assembly is then autoclaved under pressure, bonding the overlying layer to the base layer and curing a resin contained within the composite material. To remove the memory metal cores, the autoclave temperature is adjusted to heat the cores above the critical temperature of the memory metal, converting it to the austenitic state. In the austenitic state, the memory metal resumes its memory shape, wherein the cores contract to a smaller cross-sectional area. The smaller cross-sectional hollow cores are then easily removed, leaving behind an elongate void having a trapezoidal cross section. This technique is limited by the need to use hollow memory metal cores. The memory metal cores must be hollow to permit a reduction in their cross-sectional area when they revert to the memory shape. As a result, the technique has little application for forming passages of the required smaller cross section noted above.
Accordingly, it is an object of the present invention to form an elongate cavity within a component, where the cavity is of substantially greater length than possible with prior art methods. A further object is to form an elongate passage of a relatively small cross-sectional area, e.g., less than a millimeter in diameter. Yet a further object is to form cavities of a desired cross-sectional shape within a material. These and other objects and advantages of the present invention will be apparent from the attached drawings and from the Disclosure of the Preferred Embodiments that follows.